These people are being sent not into orbit, but sent up for our amusement in Channel 4's Space Cadets. Do practical jokes go too far these days?

Everybody loves a joker, right? But what if the joke is on you?

That is what nine unwitting "thrill-seekers" will eventually discover, having signed up for the experience of a lifetime - to be blasted off into space in a new reality TV series. It is, in fact, an elaborate and very expensive hoax.

The nine contestants - plus three actors planted to help the action along - are undergoing training in Russia (in reality a disused airbase in Suffolk), competing for four places on a space shuttle (a prop from Clint Eastwood's Space Cowboys film, which will be bombarded with special effects to simulate the flight).

Cue much hand wringing that reality TV has gone mad. For as the genre matures, programme makers are coming up with ever more elaborate ways to fool people.

What was it about Miriam?

Last year the reality TV show criticised for going too far was There's Something About Miriam, in which six unwitting lads wooed a stunner - who turned out to be a pre-operative transsexual.

Once the twist in Miriam's tale was revealed, the contestants cried foul and threatened legal action - Sky One only screened the show once the men had received apologies and an out-of-court settlement.

Bit of a giggle?

And now Space Cadets stands accused. Channel 4's hush-hush preparations were broken by Broadcast magazine under the headline "Is this the cruellest reality TV show yet?" Opinion would seem to be divided:

"Possibly the most audacious practical joke in television history" - Channel 4

"They are not so much the Right Stuff as the Not-So-Bright Stuff" - Daily Telegraph

An unscientific vote on this website on Wednesday asked if the concept was too nasty - the nays carried the day, but only just.

Practical jokes by their very nature involve an element of cruelty, defined as pranks intended to make the victim feel foolish. But where to draw the line between a bit of a giggle and just plain nasty?

It's a bit like a public execution - grimly fascinating, but that doesn't mean you think it should happen

Arthur Smith, comedian

Comedian Arthur Smith says that for him, drawn-out hoaxes go too far.

"It's not like being stitched up for 10 minutes or for a morning by a mate. I quite like the idea of briefly and inconsequentially fooling a friend, but writ large like this it's cruel.

"The greatest experience of your life wiped from under your feet, and you've been laughed at for weeks. I'm not saying I wouldn't want to see their faces, especially if they're irritating, but it's like a public execution - grimly fascinating, but that doesn't mean you think it should happen."

If the measure of a prank's cruelty derives from the number of people on the other side sniggering, then TV is surely the ultimate medium for playing such jokes. The man who started this genre was Allen Funt, creator of the US show Candid Camera in the late 1940s.

Comedians such as Dom Joly build careers on surreal set-ups

By today's standards his pranks were pretty tame, although, by way of acknowledging the potential for hurt, Channel 4 has taken steps to soften the inevitable blow for those taking part in Space Cadets. It has made clear that someone close to each contestant has been let in on the hoax and asked if they can take it.

In general, Smith says those who find themselves the butt of a practical joke tend to laugh so as not to seem a bad sport.

No such worries for Tom Cruise. Last June the film star was squirted with water from a fake microphone. What was intended as a bit of fun for a comedy show was an affront to his dignity: "Why would you do that? You jerk!"

Choose carefully

US comedian Tim Nyberg, author of the Practical Joke Book, says the choice of victim is important.

It can build camaraderie in the office or in a family, because there's teamwork involved

Tim Nyberg, comedian

"If it's a celebrity ripe for sending up, that adds to the entertainment, but you have to be more careful with an average Joe off the street. Our rule [when planning jokes] is nothing that causes personal injury or ends you up in a nasty lawsuit."

A well-crafted hoax can be a thing of joy, he says. "It can build camaraderie in the office or in a family, because there's teamwork involved. Used correctly, it's a good thing, otherwise it's damaging."

Those who feel aggrieved often find that some form of compensation helps them see the funny side, he says. Each space cadet, for instance, receives £5,000 for each day on set.

Psychologist Dr G Neil Martin, of Middlesex University, says practical jokes remain popular because we take vicarious pleasure in the misfortune of others.

"One theory of comedy holds that we joke to make ourselves feel superior, and laughing at those we find stupid, avaricious, unpleasant or gormless serves this function. Most reality TV contestants fall into this catch-all category. And there's an element of 'there but for the Grace of God' about enjoying practical jokes."